Tapered vs Square cut fins

[David Morrow]

Back when our motorcycles were mostly air cooled, the fins, or at least the outer most part of the cooling fins were tapered to enhance cooling. A bit of Googling seems to confirm this.

This makes me wonder why the fins on virtually every small stirling and similar engine that I've seen has square cut ends. It would seem that once the fins are cut, it wouldn't be that much trouble, with the right shaped lathe cutting tool, to put a taper on the outer end of the fins to promote better cooling.

 

[Bernd]

Back when our motorcycles were mostly air cooled, the fins, or at least the outer most part of the cooling fins were tapered to enhance cooling. A bit of Googling seems to confirm this.

Could that be because they are cast and not machined? Need the taper to get the mold out of the cavity. Unless of course they are machining them now.

 

[joeby]

There are a couple of possibilities, that immediately come to mind.
The first being that the cylinder fins on my older bikes were cast, not machined, and I think the taper may have had more to do with draft in the casting process than cooling advantages. Making the fins thinner at the tips may have allowed more airflow, true, but I'm not sure that would have been the primary concern.

Your suggestion on tapering the fins on a model may have some merit though. The fact that they normally don't have air forced through the fins as a motorcycle or airplane engine would benefit from any increase in airflow, except that, as I understand it, model engines naturally run too cold for best efficiency anyway. I may help in the appearance department if nothing else.

I, personally, would prefer the look of tapered fins (looks more like a casting that way), but the idea of adding a second operation to a sometimes nerve-racking operation outweighs the advantage for me.


Kevin (Who sees that Bernd is the faster responderer!)

 

[Mainer]

According to some things I've read in old Model Engineer magazines, the tapered fins, and especially the fillets at the root of the fins, enhance cooling. Whether that's true or not I don't know.

 

[shred]

I do know that computer heat sink fins aren't tapered. They are very thin and have hefty heat-pipes or conductive blocks, but no taper, even on the high-end ones..

 

[Bluechip]

Hi

There were, I believe, references in P. Irvings book 'Tuning for Speed' regarding superior cooling action of tapered fins on high power m/cycle engines. I no longer have the book, so I cannot confirm.

If you really want to improve thermal conduction, you can always do what AJS did on the Porcupine, as an experiment.

Cast the cylinder heads in silver ... true.

Dave

 

[Noitoen]

I would think of 2 reasons for this. Weight and material save, since the fins are cast. The reason they can be taper is (maybe) since the heat travels from the block to the tip of the fin and gradually cools on the way, they don't need constant thickness. When you look at air-conditioner conduit distribution, the cross-section grows thinner after the air outlets along the way. There is less air to distribute towards the end.
One thing though, with electronics's heat sinks is that the fins are tapered but "corrugated". this increases the area of metal exposed to air flow.

 

[Loose nut]

I don't know if tapered fins are more efficient at cooling or not but you aren't going to get the pattern out of a sand mould like they used in those days if it isn't tapered. The better efficiency may be a coincidence or even an unsubstantiated opinion but a model looks better if it is true to the prototype.

 

[GailInNM]

Cutting tapered fins by plunging between two roughed out fins with a tapered form tool does not work very well. The length of the cutting edge is large and the tool is narrow and this often leads to chatter. I tried it a time or two.

A safer way is to first rough out to square form, then set the compound for the taper wanted and realign the cutting tool to be perpendicular to the work and do one side of all the fins. Reset everything to do the second side of the fins. Lots of work, but it does give a clean finish.

For dress up purposes, a edge rounding tool can be ground up to round the edge of the fin. Some people like the look with the rounded edges.

I personally like the square look, but I build functional toys and don't often worry about the scale look.
Gail in NM,USA